[SSS15-P01] A study using waveform simulations on the applicability of seafloor strong motion records to the source process analysis
Keywords:Seafloor strong motion records, Source process analysis, Waveform simulations
We prepare 3D synthetic waveforms for models with/without a seawater and 1D synthetic waveforms at S-net stations off Fukushima for near-coast and near-trench shallow crustal earthquakes in three period bands of 5-10 s, 10-25 s, and 25-50 s. 3D synthetic waveforms for a model with a seawater are calculated with a 3D FDM simulation (Takemura et al. 2015) assuming a 3D velocity structure model, which is based on the 3D subsurface structure model of J-SHIS (Fujiwara et al. 2009, 2012) including topographies and a seawater layer. 3D synthetic waveforms for a model without a seawater are calculated assuming a 3D velocity structure model, which is same as the above model but a seawater layer is replaced with an air. 1D synthetic waveforms are calculated at each station with the discrete wavenumber method (Bouchon 1981) and the reflection/transmission matrix method (Kennett and Kerry 1979) assuming a 1D velocity structure model for each station, which is extracted from the J-SHIS model.
To investigate the effect of a seawater layer, we compare the 3D synthetic waveforms for models with/without a seawater. Although the waveforms at transverse component are almost similar, the difference between their waveforms at radial and vertical components appears at periods lower than 25 s. The distribution of stations with the waveform difference depends on the horizontal event location: the waveform differences in the near-coast event are found at stations with a deep water depth, while these in the near-trench event are shown at almost all stations. Previous studies (e.g. Nakamura et al. 2014; Noguchi et al. 2016) demonstrated that the presence of a seawater and sediments leads to the excitation of oceanic Raleigh waves and that the predominant period of the fundamental mode of the Rayleigh waves depends on the seawater thickness. Our result and previous studies suggest that in the case of the near-trench earthquakes and/or the use of waveforms at stations with a deep seawater depth, appropriate waveform components and period band for the source process analysis are limited as long as Green’s functions for a model without a seawater are used.
We also investigate how the 1D synthetic waveforms can reproduce the 3D synthetic waveforms considering the offshore heterogeneous structure. The waveform comparison suggests that although the amplitude difference and the time shift of S-wave and later phases are shown at many stations, there are stations that the phases of S-wave are similar. This suggests the possibility that seafloor records after the corrections of time shifts and amplitude differences can be used in the source process analysis.